I know its been asked a few times, with various different solutions for being able to create a digital Hydrometer for brewing beer.

After finding this page containing the basic idea that their float uses in an industrial product, it honestly doesnt seem that complex...so i am wondering if i am missing something.I dont have a lot of experience with Hall sensors and how accurate they really are...http://www.gardco.com/pages/density/electric_hydrometer.cfm

The interesting part is this

Quote

The submersible probe head contains a mathematically characterized spring-float system. When the head is immersed vertically in the liquid, the float moves vertically and the position of the float controls the position of a permanent magnet whose displacement is sensed by a concentric array of Hall-effect linear displacement sensors. The output signals of the sensors are mixed in a dedicated electronics module that provides an single output voltage whose magnitude is a direct linear measure of the quantity to be measured.

Basically its just a tube that is inserted into the fluid vertically, and a sliding tube inside of it holding a magnet is pushed up towards or away from the hall sensor based on the fluids density which will change as the beer ferments to create alcohol.

Your description raises some questions in my mind.It sounds as if the outer tube is fixed.So how do you fix it relative to the level of liquid, such that you get the correct reading?I suppose another way of saying this is how would you zero your instrument.The level of the liquid will change depending on how much liquid is in the container and will change with the temperature and density of the liquid as well as evaporation and things being added.There may also be a problem with gas bubbles and solids getting between the inner and outer tubes.Then there is the accuracy and range of the hall effect which I cannot comment on.

Not sure, from their notes it didnt sound like depth or being fixed was an issue, from the looks of the instrument y ou just hold it down into whatever you want to measure. Regardless i think going with a more static pressure differential approach will make more sense...basically have two pressure sensors at a known distance from eachother, and the pressure difference between them is directly related to the density of the liquid...I can see some sort of hollow tube(like a small PVC pipe) inserted/held vertically into the liquid with a few pressure sensors spaced apart. Figuring out how to setup such a thing so that the liquid cant get back into the pressure sensor will be a bit more difficult, i had read somewhere people recommend using tubing, and just looping it a few times, which should prevent the liquid from being able to get in. Another idea is a piezo pressure sensor, but i dont know if you could just coat those with something waterproof if they would function properly...or read accurately and precisely enough.

Not sure, from their notes it didnt sound like depth or being fixed was an issue

Depth is not an issue, temperature is and there are fudge factors to compensate...if you know the temperature at the time of reading. also, gas bubbles may be a problem as rarman stated. Even plain old sealed tube hydrometers are sometimes a pain to use. I used them for wine, but never for beer, and never with an actively fermenting solution.

The point is; unless you are trying to automate the process of taking hydrometer readings, and maybe logging them, It's easier just to buy an ordinary hydrometer.if you are going to automate readings then A Hall sensor sounds like it will do the job. Remember though, any hydrometer ONLY measures density, it does not directly measure alcohol content, though that may be the major contributing chemical in the solution.